import pandas as pd
df = pd.DataFrame({'商品':['苹果','香蕉','梨'],'单价（元）':[5,5,4],'销量(kg)':[100,None,None],'库存(kg)':[50,None,70]})
print(df)
print(df.dropna())
print(df.dropna(axis=1))
print(df.dropna(thresh=3))
print(df.dropna(subset=['库存(kg)']))